Crack control for substrate separation

ABSTRACT

A method for separating a layer for transfer includes forming a crack guiding layer on a substrate and forming a device layer on the crack-guiding layer. The crack guiding layer is weakened by exposing the crack-guiding layer to a gas which reduces adherence at interfaces adjacent to the crack guiding layer. A stress inducing layer is formed on the device layer to assist in initiating a crack through the crack guiding layer and/or the interfaces. The device layer is removed from the substrate by propagating the crack.

BACKGROUND

1. Technical Field

The present invention relates to wafer transfer, and more particularlyto methods for transferring layers or substrates by controlling crackpropagation.

2. Description of the Related Art

Wafer transfer processing is employed to transfer layers from onesubstrate to another. There is interest in processes that can separate adevice layer from an underlying single crystal base substrate whileleaving the base substrate smooth enough to not require a significantamount of polishing before the substrate is used again. In one process,an epitaxial lift-off procedure utilizes an AlAs separation layerbetween a base substrate (e.g., Ge or GaAs) and a III-V epitaxial devicelayer and separates the epitaxial layer from the base substrate layer bylaterally etching the AlAs layer with HF. This approach is verytime-consuming to the point that it is not practical for manufacturing,especially for large area wafers.

In another process, spalling is employed. In this approach, a metallicstress layer is deposited on a layer stack that includes a substrate,epitaxial buffer layer and a III-V device layer. The structure is thencracked by splitting the buffer layer, followed by a selective etch toremove buffer layer residuals from either side of the crack face.However, the depth of the crack can be difficult to control and itcannot always be confined to the buffer layer.

SUMMARY

A method for separating a layer for transfer includes forming a crackguiding layer on a substrate and forming a device layer on thecrack-guiding layer. The crack guiding layer is weakened by exposing thecrack-guiding layer to a gas which reduces adherence at interfacesadjacent to the crack guiding layer. A stress inducing layer is formedon the device layer to assist in initiating a crack through the crackguiding layer and/or the interfaces. The device layer is removed fromthe substrate by propagating the crack.

Another method for separating a layer for transfer includes growing acrack guiding layer on a monocrystalline substrate; forming a devicelayer on the crack-guiding layer, the device layer including crystallineIII-V material; etching the crack guiding layer at exposed portions at aperiphery to form a recess crack that assists in crack formation;weakening the crack guiding layer by exposing the crack-guiding layer toa gas which reduces adherence at interfaces adjacent to the crackguiding layer; forming a stress inducing layer on the device layer tofurther assist in initiating a crack through the crack guiding layerand/or the interfaces; and removing the device layer from the substrateby propagating the crack.

Yet another method for separating a layer for transfer includes growinga crack guiding layer on a monocrystalline substrate, the crack guidinglayer including AlAs; forming a device layer on the crack-guiding layer;etching the crack guiding layer at exposed portions at a periphery toform a recess crack that assists in crack formation; weakening the crackguiding layer by exposing the crack-guiding layer to an oxidation agentwhich reduces adherence at interfaces adjacent to the crack guidinglayer; forming a stress inducing layer on the device layer to furtherassist in initiating a crack through the crack guiding layer and/or theinterfaces; and removing the device layer from the substrate bypropagating the crack.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view of a substrate employed in accordancewith the present principles;

FIG. 2A is a cross-sectional view of the substrate of FIG. 1 having acrack guiding layer formed thereon in accordance with the presentprinciples;

FIG. 2B is a cross-sectional view of the substrate of FIG. 1 having aprotection layer and a crack guiding layer formed thereon in accordancewith the present principles;

FIG. 3 is a cross-sectional view of the substrate of FIG. 2A having adevice layer formed on the crack guiding layer in accordance with thepresent principles;

FIG. 4 is a cross-sectional view of the substrate of FIG. 3 having astress-inducing layer formed on the device layer in accordance with thepresent principles;

FIG. 5 is a cross-sectional view of the substrate of FIG. 4 showing thecrack guiding layer recessed by etching in accordance with the presentprinciples;

FIG. 6 is a cross-sectional view of the substrate of FIG. 5 showing acrack being propagated along the crack guiding layer and its interfaceswith adjacent layers in accordance with the present principles;

FIG. 7 is a cross-sectional view showing a mechanism for weakening thecrack guiding layer in accordance with the present principles;

FIG. 8 is a cross-sectional view showing the device layer separated fromthe crack guiding layer and a holder or handling substrate adhered tothe stress-inducing layer in accordance with the present principles;

FIG. 9 is a cross-sectional view showing the substrate ready for reuseand the device layer ready for transfer to another layer or substrate inaccordance with the present principles; and

FIG. 10 is a block/flow diagram showing a method for separating a layerfor transfer in accordance with illustrative embodiments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with the present principles, a layer release processemploys a stress layer, but utilizes a chemical reaction-induceddegradation of adhesion of an epitaxial buffer layer. Theadhesion-degrading reaction of the epitaxial buffer layer may beperformed after an initial lateral recess etch or without an initiallateral recess etch. Layer separation is then induced by a spontaneouspeeling at a weak interface rather than by the propagation of a crack orfracture, characteristic of spalling. In one embodiment, an AlAs layeror similar layer is employed and is oxidized by air, moisture, H₂O₂, O₂or other compounds. For example, when AlAs is oxidized to AlO_(x), theAs will be “kicked out” to an interface with adjacent layers. This willweaken the interface and lead the crack to propagate along the AlAslayer. It should be noted that other materials and adherence reducinggas environments may also be employed.

In another embodiment, a crack guiding layer is inserted between asubstrate and a device layer. The crack guiding layer may be selectivelyetched to partially recess the crack guiding layer relative to adjacentlayers. The crack guiding layer may serve as a crack initiation controlto determine and control a position for a cleavage interface. Then, astress layer may be applied on the structure to from the crack, whichcan propagate along two interfaces of the crack guiding layer (e.g.,AlAs). This process may be performed in the air or in an oxidizingenvironment to increase the oxidation rate of the crack guiding layer(to reduce adhesion). Materials other than those described in theexamples may also be employed.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

A design for an integrated circuit chip or photovoltaic device may becreated in a graphical computer programming language, and stored in acomputer storage medium (such as a disk, tape, physical hard drive, orvirtual hard drive such as in a storage access network). If the designerdoes not fabricate chips or the photolithographic masks used tofabricate chips, the designer may transmit the resulting design byphysical means (e.g., by providing a copy of the storage medium storingthe design) or electronically (e.g., through the Internet) to suchentities, directly or indirectly. The stored design is then convertedinto the appropriate format (e.g., GDSII) for the fabrication ofphotolithographic masks, which typically include multiple copies of thechip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips or photovoltaic devices. The resulting chips/devices canbe distributed by the fabricator in raw wafer form (that is, as a singlewafer that has multiple unpackaged chips), as a bare die, or in apackaged form. In the latter case, the chip/device is mounted in asingle chip package (such as a plastic carrier, with leads that areaffixed to a motherboard or other higher level carrier) or in amultichip package (such as a ceramic carrier that has either or bothsurface interconnections or buried interconnections). In any case, thechip/device is then integrated with other chips, discrete circuitelements, and/or other signal processing devices as part of either (a)an intermediate product, such as a motherboard, or (b) an end product.The end product can be any product that includes integrated circuitchips or photovoltaic devices, ranging from toys, energy collectors,solar devices and other applications including computer products ordevices having a display, a keyboard or other input device, and acentral processor. The photovoltaic devices described herein areparticularly useful for solar cells or panels employed to provide powerto electronic devices, homes, buildings, vehicles, etc. The photovoltaicdevices may be large scale devices on the order of feet or meters inlength and/or width, or may be small scale devices for use incalculators, solar powered lights, etc.

It should also be understood that material compounds will be describedin terms of listed elements, e.g., AlAs, GaAs or InGaAs. These compoundsinclude different proportions of the elements within the compound, e.g.,InGaAs includes In_(x),Ga_(y)As_(1−x−y), where x, y are less than orequal to 1, or AlAs includes Al_(x)As_(1−x) where x is less than orequal to 1, etc. In addition, other elements may be included in thecompound, such as, e.g., AlInGaAs, and still function in accordance withthe present principles. The compounds with additional elements will bereferred to herein as alloys.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a substrate 12 or a stackof layers including or not including a substrate is provided. For easeof explanation, the substrate 12 will be described through the processflow. The substrate 12 may include a single crystal (monocrystalline)substrate. The substrate 12 may include, e.g., Ge, GaAs, InGaAs, InP,SiGe, Si, or other materials and may include an on-axis or off-cutwafer.

Referring to FIG. 2A, a crack guiding layer 14 is formed on thesubstrate 12. The crack guiding layer 14 may include a thickness ofbetween about 100 nm to about 5 microns, although other thicknesses canbe employed. Crack guiding layer 14 may include AlAs, AlGaAs, InAlAs, Geor other materials that function in accordance with the presentprinciples.

Referring to FIG. 2B, in one embodiment, in addition to the crackguiding layer 14, a protection layer 16 is formed on the substrate 12.The protection layer 16 is configured to protect the substrate 12 fromdamage during processing. The protection layer 16 may be epitaxiallydeposited on the substrate 12 and may include a monocrystallinesubstance, such as Ge or the like. Other materials for protection layer16 may also be employed, such as, other monocrystalline materials (e.g.,InGaAs, etc.), dielectric materials, etc. depending on the othermaterial choices for the substrate 12 or other layers (crack guidinglayer 14), etc. to be employed in the method. The protection layer 16need not be thick, which will reduce processing time for its removal.The protection layer 16 may include a thickness of about 0.5 microns ofless, although other thicknesses may be employed.

The crack guiding layer 14 is formed on the protection layer 16 and mayinclude a thickness of between about 100 nm to about 5 microns, althoughother thicknesses can be employed. Crack guiding layer 14 may includeAlAs, AlGaAs, InAlAs, Ge, etc.

Referring to FIG. 3, a device layer 18 is formed on the crack guidinglayer 14. The protection layer 16 may be employed, but is not shown. Thedevice layer 18 may include any material to be transferred to anothersubstrate or layer. In one embodiment, the device layer 18 includes aIII-V material. The III-V material may be monocrystalline. The devicelayer 18 is depicted as a single layer but may include multiple layers.The device layer 18 may be epitaxially grown or otherwise deposited onthe crack guiding layer 14. In one embodiment, device layer 18 includesa layer that is lattice-matched with the crack guiding layer 14. Thedevice layer 18 may be employed in the fabrication of integratedcircuits, field effect transistor devices, photovoltaic cells, etc.

Referring to FIG. 4, a stress inducing layer 20 is formed on the devicelayer 18 over the entire wafer. The stress inducing layer 20 may includea metal, such as Ni, or may include a semiconductor material, such asSiGe, etc. Other materials may also be employed, such as W or Co.

Referring to FIG. 5, in one embodiment, exposed edges of the crackguiding layer 14 may be etched with an appropriate etchant to pre-form acrack by forming recesses 22. In one embodiment, HF is employed as anetchant to etch, e.g., AlAs, although other etchants may be employed.

Referring to FIG. 6, a crack 24 is initiated by pulling or lifting anedge or from the stress of the stress inducing layer 20, which can behigh enough to cause crack propagation. In one embodiment, the crackguiding layer 14 may be exposed to an oxidizing atmosphere to weaken theinterfaces of the crack guiding layer 14 with the substrate 12 and/orthe device layer 18. The oxidizing atmosphere may include exposure toair, steam, H₂O₂ or other oxidant to enhance the oxidation of the crackguiding layer 14, e.g., AlAs, and weaken the interface(s). The crackpropagation is limited to the crack guiding layer 14 or the interfacesadjacent to the crack guiding layer 14.

Referring to FIG. 7, an explanation of the weakening mechanism isillustratively described for a cracking guidance layer 14 made of AlAs.Other materials may be employed as well. The AlAs is oxidized by air,moisture, H₂O₂, O₂ or other compounds. When AlAs is oxidized, AlO, isformed, and the liberated As moves toward the interfaces with adjacentlayers. This weakens the interface and leads the crack to propagatealong the cracking guidance layer 14 and/or the interfaces.

Referring to FIG. 8, after separating the substrate 12 from the devicelayer 18 using the crack guiding layer 14, the crack guiding layer 14 isremoved from the substrate 12. If a protection layer (16) is employed,the protection layer may be removed as well.

The crack guiding layer 14 may be etched using HF or other etchant. Thedevice layer 18 may be transferred to other holders or substrates 26,such as glass, metal, etc. for the following fabrication steps. Itshould be understood that a holder or substrate 26 may be attached tothe stress inducing layer 20 prior to the crack propagation/separationstep.

In FIG. 9, separated portions are shown with the substrate 12 ready forreuse for a next process, and the device layer 18 ready for transfer toanother layer, stack of layers or substrate.

Referring to FIG. 10, a method for separating a layer for transfer isillustratively shown. It should be noted that, in some alternativeimplementations, the functions noted in the blocks may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

In block 102, a protection layer is optionally formed on a substrate. Inone embodiment, the protection layer includes Ge on a GaAs substrate. Inblock 104, a crack guiding layer is formed on the substrate. The crackguiding layer may include, e.g., AlAs and the device layer may include,e.g., a III-V material. These layers may be formed by employing anepitaxial deposition process.

In block 106, a device layer is formed on the crack guiding layer. Inblock 108, the crack guiding layer may be etched at exposed portions ata periphery of the device stack to pre-form a crack. The crack guidinglayer is recessed to form a natural stress riser. In block 110, thecrack guiding layer is weakened by exposing the crack-guiding layer to agas which reduces adherence at interfaces adjacent to the crack guidinglayer. Weakening the crack guiding layer may include exposing the crackguiding layer to oxygen or a compound including oxygen. This isparticularly useful when the crack guiding layer includes AlAs.

In block 112, a stress inducing layer is formed on the device layer toassist in initiating a crack through the crack guiding layer and/or theinterfaces. The stress inducing layer may include depositing Ni, SiGe,W, Co or other material. In block 114, a holder or substrate may beadhered to the stress inducing layer to enable transport fortransferring the device layer, once removed. In block 116, the devicelayer is removed from the substrate by propagating the crack. In block118, the substrate may be reused for further processing after the crackguiding layer is removed. Processing can continue.

Having described preferred embodiments for crack control for substrateseparation (which are intended to be illustrative and not limiting), itis noted that modifications and variations can be made by personsskilled in the art in light of the above teachings. It is therefore tobe understood that changes may be made in the particular embodimentsdisclosed which are within the scope of the invention as outlined by theappended claims. Having thus described aspects of the invention, withthe details and particularity required by the patent laws, what isclaimed and desired protected by Letters Patent is set forth in theappended claims.

What is claimed is:
 1. A method for separating a layer for transfer,comprising: forming a crack guiding layer on a substrate; forming adevice layer on the crack-guiding layer; weakening the crack guidinglayer by exposing the crack-guiding layer to a gas which reducesadherence at interfaces adjacent to the crack guiding layer; forming astress inducing layer on the device layer to assist in initiating acrack through the crack guiding layer and/or the interfaces; andremoving the device layer from the substrate by propagating the crack.2. The method as recited in claim 1, wherein the crack guiding layerincludes AlAs and the device layer includes a III-V material.
 3. Themethod as recited in claim 1, wherein weakening the crack guiding layerincludes exposing the crack guiding layer to oxygen or a compoundincluding oxygen.
 4. The method as recited in claim 1, furthercomprising etching the crack guiding layer at exposed portions on aperiphery to pre-form the crack.
 5. The method as recited in claim 1,wherein forming the stress inducing layer includes depositing one of Ni,W, Co and SiGe.
 6. The method as recited in claim 1, further comprisingforming a protection layer on the substrate before forming the crackguiding layer.
 7. The method as recited in claim 1, further comprisingapplying a holder substrate to the device layer to enable transport fortransferring the device layer.
 8. The method as recited in claim 1,further comprising reusing the substrate for further processing.
 9. Amethod for separating a layer for transfer, comprising: growing a crackguiding layer on a monocrystalline substrate; forming a device layer onthe crack-guiding layer, the device layer including crystalline III-Vmaterial; etching the crack guiding layer at exposed portions at aperiphery to form a recess crack that assists in crack formation;weakening the crack guiding layer by exposing the crack-guiding layer toa gas which reduces adherence at interfaces adjacent to the crackguiding layer; forming a stress inducing layer on the device layer tofurther assist in initiating a crack through the crack guiding layerand/or the interfaces; and removing the device layer from the substrateby propagating the crack.
 10. The method as recited in claim 9, whereinthe crack guiding layer includes AlAs.
 11. The method as recited inclaim 9, wherein weakening the crack guiding layer includes exposing thecrack guiding layer to oxygen or a compound including oxygen.
 12. Themethod as recited in claim 9, wherein forming the stress inducing layerincludes depositing one of Ni, W, Co and SiGe.
 13. The method as recitedin claim 9, further comprising forming a protection layer on thesubstrate before forming the crack guiding layer.
 14. The method asrecited in claim 9, further comprising applying a holder substrate tothe device layer to enable transport from transferring the device layer.15. The method as recited in claim 9, further comprising reusing thesubstrate for further processing.
 16. A method for separating a layerfor transfer, comprising: growing a crack guiding layer on amonocrystalline substrate, the crack guiding layer including AlAs;forming a device layer on the crack-guiding layer; etching the crackguiding layer at exposed portions at a periphery to form a recess crackthat assists in crack formation; weakening the crack guiding layer byexposing the crack-guiding layer to an oxidation agent which reducesadherence at interfaces adjacent to the crack guiding layer; forming astress inducing layer on the device layer to further assist ininitiating a crack through the crack guiding layer and/or theinterfaces; and removing the device layer from the substrate bypropagating the crack.
 17. The method as recited in claim 16, whereinforming the stress inducing layer includes depositing one of Ni, W, Coand SiGe.
 18. The method as recited in claim 16, further comprisingforming a protection layer on the substrate before forming the crackguiding layer.
 19. The method as recited in claim 16, further comprisingapplying a holder substrate to the device layer to enable transport fromtransferring the device layer.
 20. The method as recited in claim 16,further comprising reusing the substrate for further processing.